Several mutations have been induced by n-ethyl-n-nitrosourea in mice and detected by the biochemical specific locus test at the MOD-1 and GPI loci. The frequency of MOD-1 mutations is much greater than the frequency of GPI mutations. The objectives of this work are to (1) complete a comparative biochemical analysis of induced mutations of MOD-1 and GPI in mice, (2) analyze at the molecular level the relationship of sequence, chromatin structure, and regulation to the frequency and location of induced DNA lesions, and (3) evaluate whether or not any of the induced mutations can be used as markers for mutational events directly in exposed animals.